I have a water softener contractor who runs his back wash line to a "curtain drain" French drain outside the home for people on private wells. We use the 2015 IPC and 802.1.5 requires an air gap or an air break. This contactor is complaining that he wants to have a direct connection from the softener backwash line to the curtain drain because the unit is backing up through the air gap fitting. He wants to put in a check valve and says it's not dangerous waste. Anyone have any insight on this?

The air gap is generally 1-1/2 times the diameter of the drain tube.
Best Wishes

What's the purpose of the air gap? I have a water softner installed by Culligan which dumps to a basement floor drain with an air gap. Same drain has first floor kitchen sink draining into it with an 8" air gap. Is this to eliminate backing up into kitchen if the sewer drain clogs?

IT is to prevent sewer water from contaminating the water system if the sewer line backs up. Dumping to a "French drain" which may be a hole in the ground filled with stones, would back up because the water could not enter the ground as fast as the softener is draining.

Yes, and the IPC references 802.1.5 (2015 IPC) the building drainage system and this is a French drain which is not the building drainage system but appears to have a potental hazard of back up. The contactor claims this could not back up because it's a private well and no combination of things could create backflow or back syphonage. I'm trying to error on the side of public health protection but don't want to require something that is un-needed overkill.